POWER SPECTRA OF THE EDDY-VELOCITY COMPONENTS

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  • 1 Massachusetts Institute of Technology
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Abstract

Power spectra of the eddy-velocity components have been determined at four levels within the layer from 2 to 12 meters under varying conditions of mean wind speed, trajectory and thermal stability. A filtering technique suggested by J. W. Tukey has been used to obtain rough estimates of contributions to the total variance for seven consecutive frequency intervals within the range from about 0.5 to 0.005 cycles per second. At the higher frequencies studied, variances for all three components are approximately equal and equipartition of turbulent energy is thus indicated. Spectra for the u- and v-components appear to be invariant with frequency at the lowest level, and tend to increase slowly with decreasing frequency at the higher levels. The w-spectra at all levels fall off sharply with decreasing frequency, contributions to the vertically-directed energy becoming almost negligible at the lowest frequencies investigated.

Abstract

Power spectra of the eddy-velocity components have been determined at four levels within the layer from 2 to 12 meters under varying conditions of mean wind speed, trajectory and thermal stability. A filtering technique suggested by J. W. Tukey has been used to obtain rough estimates of contributions to the total variance for seven consecutive frequency intervals within the range from about 0.5 to 0.005 cycles per second. At the higher frequencies studied, variances for all three components are approximately equal and equipartition of turbulent energy is thus indicated. Spectra for the u- and v-components appear to be invariant with frequency at the lowest level, and tend to increase slowly with decreasing frequency at the higher levels. The w-spectra at all levels fall off sharply with decreasing frequency, contributions to the vertically-directed energy becoming almost negligible at the lowest frequencies investigated.

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